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Date:   Wed, 23 Nov 2022 13:02:27 +0100 (CET)
From:   Anna-Maria Behnsen <anna-maria@linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
cc:     LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Julia Lawall <Julia.Lawall@inria.fr>,
        Arnd Bergmann <arnd@arndb.de>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Marc Zyngier <maz@kernel.org>,
        Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        linux-bluetooth@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org
Subject: Re: [patch V2 15/17] timers: Provide timer_shutdown[_sync]()
In-Reply-To: <20221122173648.962476045@linutronix.de>
Message-ID: <3779da12-6da5-8f6b-ec93-f8d52e38a40@linutronix.de>
References: <20221122171312.191765396@linutronix.de> <20221122173648.962476045@linutronix.de>
MIME-Version: 1.0
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Precedence: bulk

On Tue, 22 Nov 2022, Thomas Gleixner wrote:

> @@ -1605,6 +1629,48 @@ int timer_delete_sync(struct timer_list
>  }
>  EXPORT_SYMBOL(timer_delete_sync);
>  
> +/**
> + * timer_shutdown_sync - Shutdown a timer and prevent rearming
> + * @timer: The timer to be shutdown
> + *
> + * When the function returns it is guaranteed that:
> + *   - @timer is not queued
> + *   - The callback function of @timer is not running
> + *   - @timer cannot be enqueued again. Any attempt to rearm
> + *     @timer is silently ignored.
> + *
> + * See timer_delete_sync() for synchronization rules.
> + *
> + * This function is useful for final teardown of an infrastructure where
> + * the timer is subject to a circular dependency problem.
> + *
> + * A common pattern for this is a timer and a workqueue where the timer can
> + * schedule work and work can arm the timer. On shutdown the workqueue must
> + * be destroyed and the timer must be prevented from rearming. Unless the
> + * code has conditionals like 'if (mything->in_shutdown)' to prevent that
> + * there is no way to get this correct with timer_delete_sync().
> + *
> + * timer_shutdown_sync() is solving the problem. The correct ordering of
> + * calls in this case is:
> + *
> + *	timer_shutdown_sync(&mything->timer);
> + *	workqueue_destroy(&mything->workqueue);
> + *
> + * After this 'mything' can be safely freed.
> + *
> + * This obviously requires that the timer is not required to be functional
> + * for the rest of the shutdown operation.

NIT... Maybe the first requires could be replaced by
assumes/expects/presupposes to prevent double use of required?

Thanks,

	Anna-Maria